Regulus (Alpha Leonis)

Regulus (Alpha Leonis) is the brightest star in the constellation Leo and the 21st brightest star in the sky as seen from Earth. Regulus is a
bluish-white B star; its Latin name means
"the little king" – the reference to a kingly star going back to ancient
times. Regulus marks the bottom of the Sickle that outlines
the Lion's head, and lies almost exactly on the ecliptic so that it is regularly occulted by
the Moon.

Regulus is a main sequence star like the
Sun that generates light by converting hydrogen into helium at its core.
However, Regulus is more massive, so it shines brighter, hotter, and bluer.
With a spectral type of B7 and a distance of 77 light-years, it's the closest
B-type main-sequence star to the Sun.

Regulus is orbited by a distant binary pair – an orange dwarf and
a red dwarf. These fainter, lower-mass companions go around Regulus at a
distance of about 4,200 AU and with a period of at least 130,000 years.
They are themselves separated by at least 95 AU in a 1,000-year orbit. Both
these companions of Regulus are less massive and dimmer than the Sun.

In 2008, Douglas Gies of Georgia State University and colleagues found that
Regulus also has a much closer, faint companion. This unseen star gave away
its presence through its gravitational pull, which causes Regulus to wobble
to and fro. The companion has a third of the Sun's mass, orbits Regulus
every 40.1 days and is only 52 million km it – slightly closer than
Mercury is to the Sun. It seems likely that it is a white dwarf.

Fast spin and flattened shape

Regulus has an extremely rapid rate of rotation. Its equatorial rotation
velocity is 317 km/s (709,000 mph), compared with the Sun's 2 km/s (4,470
mph).1 Whereas Sun takes a month to spins once around, Regulus
dos the same in 15.9 hours. If Regulus spun just 10% faster, researchers
estimate, it would fly apart.

This rapid rotation flattens Regulus into a pumpkin shape. Its equatorial
diameter is 32% greater than its polar diameter: the former is 1.65 milliarcseconds
and the latter 1.25 milliarcseconds. At Regulus' distance, these numbers
translate into actual equatorial and polar diameters of 4.16 and 3.14 solar
diameters, respectively. The distortion results in a big difference in surface
temperature of the star between poles (15,400 K) and equator (10,300 K).
Furthermore, because the poles are so hot, the star shines brightest there.
The equator radiates only 20% as much light per square inch as the poles
do. Previously, this phenomenon of gravity darkening had been seen only
in the case of eclipsing binaries.

The same team that discovered the close companion star in orbit around Regulus
also suggested it was the source of the primary's flattening. Material shed
by the companion during its red giant phase may have been dumped onto Regulus,
giving rise to a large equatorial bulge.

visual magnitude

1.36

absolute magnitude

-0.52

spectral type

B7V + K1-2V + M5V

surface temp. (mean)

12,000 K

luminosity

240 Lsun

radius (mean)

3.5 Rsun

mass

3.4 Msun

age

~ 50 million yr

distance

77.5 light-years (23.8 pc)

position

R.A. 10h 08m 22.3s,
Dec. +11° 58' 02"

Other fast spinners

Regulus isn't the first bright star found to be extremely flattened because
of its rapid spin. In 2001, astronomers reported that fast-spinning Altair,
a white star of spectral type A7, has an equatorial diameter 14% greater
than its polar diameter. And in 2003, astronomers discovered the equatorial
diameter of the blue B3 main sequence star Achernar is 56% greater than its polar diameter, making it flattest star known.